1. Field of the Invention
The disclosed and claimed concept relates to a circuit breaker and, more specifically, to a contact wear indicator assembly that is structured to provide an indication of contact characteristics and contact wear for enclosed contacts such as, but not limited to, contacts in a vacuum circuit interrupter.
2. Background Information
Circuit breaker assemblies (CBA) provide protection for electrical systems from electrical fault conditions such as current overloads, short circuits, and low level voltage conditions. Typically, circuit breakers include a spring-powered operating mechanism which opens electrical contacts to interrupt the current through the conductors in an electrical system in response to abnormal conditions. In particular, vacuum circuit interrupters include separable main contacts disposed within an insulated and hermetically sealed housing. Generally, one of the contacts is fixed relative to both the housing and to an external electrical conductor. The other contact is moveable. In the case of a vacuum circuit interrupter, the moveable contact assembly usually comprises a stem of circular cross-section having the contact at one end enclosed within the vacuum chamber, and an operating mechanism at the other end which is external to the vacuum chamber. The operating mechanism includes compression springs operatively coupled to a pole shaft which is, in turn, operatively coupled to the movable contact. When the operating mechanism is actuated, either manually or in response to an over-current condition, the moveable contacts are separated from the fixed contacts.
Over time the contacts are subject to wear and tear which diminishes the thickness of the contact assembly contact plates. These plates are disposed in the vacuum chamber and cannot be seen. While there are tools and methods to determine contact wear, see, e.g. U.S. Pat. No. 6,150,625, issued Nov. 21, 2000, such constructs are not incorporated into the circuit breaker and cannot interact therewith. Further, such constructs provide for a determination of contact wear only when used and do not provide continuous check on the contact characteristics. The contacts may wear between checks performed during the scheduled maintenance or downtime.
Further, the wear affects the contact wipe. That is, the contact “wipe” is the force holding the vacuum interrupter contacts closed and the energy available to hammer the contacts open with sufficient speed for reliable interruption. A related measurement is the contact “stroke.” Stroke is the gap between fixed and moving contacts of a vacuum interrupter when the circuit breaker contacts are in the open, first position. The circuit breaker operating mechanism provides a fixed amount of motion to the second contact assembly. The first portion of the motion is used to close the contacts (i.e., stroke) and the remainder is used to further compress a preloaded wipe spring. This additional compression is called wipe. Wipe and stroke are thus generally related to each other by relation: total displacement=stroke+wipe. Thus, as the stroke increases due to the erosion of contacts, the wipe decreases.
Accordingly, there is a need for a contact wear indicator assembly to measure contact wear that is incorporated into the circuit breaker and can interact therewith.
There is a further need for a contact wear indicator assembly that constantly measures contact wear, stroke and wipe; and communicates to the switchgear or control room during each circuit breaker operation for continuous monitoring of the contact health.
There is a further need for a contact wear indicator assembly to be compatible with existing circuit breakers.